1. Field of the Invention
The present invention relates to electrical connectors and, more particularly, is directed toward a telephone-style modular plug that can operate at higher frequencies with lower crosstalk.
2. Description of Related Art
Data communication systems being developed are constantly requiring higher and higher transmission rates. As the rates have increased to the 100 Megahertz (MHz) range, the problem of near end crosstalk (NEXT) has become particularly vexing. Crosstalk refers to the signals induced in an adjacent conductor due to magnetic (inductive) and electric field (capacitive) coupling between the conductors. The crosstalk of interest to this invention occurs in telephone-style modular plugs (near end crosstalk or "NEXT"). The crosstalk in cables and modular jacks are related fields but are not specifically addressed by this invention.
Advances in cable design and improved control of manufacturing processes have improved the electrical performance of network data cables from -32 dB NEXT to better than -42 dB NEXT at a transmission frequency of 100 MHz. This is a dramatic improvement in isolating the coupling of a signal being transmitted through cables, especially those carrying eight conductors twisted together in pairs as used in the telecommunications industry. As a result of these advances in cable electrical performance, the performance of prior art modular plugs has fallen farther behind, so that the amount of crosstalk within a modular plug has become the most significant limiting factor in a system of networking cables, female modular jacks or Alto outlets, and male modular plugs. A large part of the problem arises when the conductors leave the protective confines of the cable jacket, even though crosstalk is minimized by the conductors remaining twisted together in pairs. In order to terminate the conductors in a telephone-style modular plug, however, they must be untwisted and mounted in the plug's dielectric housing in a substantially parallel arrangement, a condition wherein the conductors are most susceptible to NEXT.
NEXT is the electrical field generated by a signal which is transmitted into a first connection, and this electrical field has lines of force which pass around and into a second connection, causing an electrical signal to flow in the second connection. This induced electrical signal flow alters and acts upon any original transmitted signal sent through the second connection, with the outcome that any receiver of the second connection signal sees an altered, distorted signal. This is the source of signals that cannot be correctly understood and therefore requires that the original second transmitted signal be transmitted again, using up valuable data bandwidth and degrading the performance of a connection system. As crosstalk becomes increasingly larger, it can have the same signal strength as the original transmitted signal, rendering the entire connection useless because it is impossible to separate the induced signal (crosstalk) from the original signal. This is commonly referred to as S/N, or signal to noise ratio. If the noise (crosstalk) is as strong as the signal, then it is impossible to separate the original signal from the induced signal (noise). The reduction of crosstalk is extremely important to enable connection systems to transmit signals as error free as possible, and to increase the data frequency that a connection system can deliver with more signal than noise.
A number of years ago, a standards committee comprised of representatives of various companies and organizations in the electronics, computer, and telecommunications industries began the development of a voluntary standard called EIA/TIA 568. The objective of this standard was to provide for interchangeability between various manufacturers' components and to set forth a minimum set of electrical requirements needed to deliver a usable signal at frequencies up to 100 MHz independently of which manufacturer's products might be used in a networking connection system. This standard was completed only in the last few years and sets out mechanical and dimensional requirements for modular female jacks/outlets, and for modular male plugs to assure mating compatibility. This so-called 568 standard also defines a set of minimum electrical requirements for cables, for modular male plugs, and for modular female jacks/outlets at various frequencies from 0.772 MHz to 100 MHz for products classified into categories. For example, the electrical requirements for category 3 components is less stringent than the electrical requirements for category 5 Ad components. This standard also specifies the conductor wiring arrangements within the male plugs, distance limitations for cable and for cable assemblies terminated with modular plugs.
Referring now to the electrical requirements of EIA/TIA 568, it sets out the minimum NEXT for any one conductor pair to any other conductor pair within the cable, as well as within the male plug as terminated onto a section of cable. Inasmuch as modular plugs are relatively small in size, it is inevitable that the close proximity of the contacts and terminated ends of the conductors induce crosstalk between different signal pairs. The most crosstalk allowed for a category 5 modular plug between worst case pairs is -40 dB at 100 MHz. As category 5 cables generally have four conductor pairs, the worst case is those two conductor pairs that have the most crosstalk to each other and more crosstalk than any other two conductor pairs. Because of the wiring arrangement specified by EIA/TIA 568, the worst case pairs are always from pair 1, corresponding to contact positions in the plug of 4 and 5, measured to pair 2, corresponding to contact positions in the plug of 3 and 6 (see the wiring arrangements of FIGS. 1 and 2). This interleaved wiring arrangement creates a high level of crosstalk within the conductor wiring exposed in the plug.
Various approaches have been used to try and overcome these NEXT deficiencies in the design of the plug. As stated before, NEXT is a function of inductive and capacitive interactions between conductors. The general thrust of the industry is to address only the capacitive problems. Rohrbaugh et al., in U.S. Pat. No. 5,628,647, seek to reduce both the magnetic and capacitive coupling by utilizing the feature of staggering or offsetting conductor receiving channels, but the remainder of the most pertinent related art concentrate solely on the capacitive effects. For example, Kristiansen in his U.S. Pat. No. 5,284,447 forms an elongated aperture in the body of the contact terminals, thus reducing the capacitance between adjacent contact terminals by reducing the amount of their confronting surface areas. U.S. Pat. No. 5,593,314 to Lincoln teaches a structure which staggers the longitudinal location of the confronting bodies of the contact terminals to reduce their capacitance. U.S. Pat. No. 5,727,962, to Caveney et al. teaches the offset terminal end arrangement disclosed in Rohrbaugh et al., supra, and forces the cable into the modular plug as far as possible, so that the length of untwisted conductors will be as short as possible.
All of these prior art patents, specifically incorporated herein by reference, are successful in what they do, but they limit their concerns solely to the electrically conducting components, namely, to the arrangement of the conductors and the structure of the terminal contacts. The instant invention, in contrast, extends this inventive field to include the body of the modular plug.
Undesirable near end crosstalk between conductors is primarily a function of capacitance: the more the capacitance, the more the crosstalk. Thus, in order to reduce the NEXT, the capacitance between the conductors must be reduced. Capacitance is dependent on two factors: (1) it is inversely proportional to the center-to-center distance between the conductors; and (2) it is directly proportional to the dielectric constant of all of the matter surrounding the conductors. Consequently, increasing the distance between the primary conductors lowers the capacitance, and lowering the average dielectric constant in the vicinity of the conductors also lowers the capacitance.
The primary area of interest of the present invention is the reduction of the effective dielectric constant of the material surrounding the conductors, i.e., the average dielectric constant of all of the materials which are present.
While the recent prior art makes some improvement toward addressing the problem of NEXT within the plug as assembled onto the cable, it remains deficient in significantly improving NEXT in the critical transition area of the plug where the conductors leave the controlled structure of the jacketed cable and are exposed to each other in a confined environment prior to their point of termination by the contact blades.